Electrical induction motors are used in many systems, such as pumps, which are subject to underloading and overloading conditions. A water pump, for example, may become underloaded when the level of water falls below the pump intake valve and may become overloaded when sand or other debris present within the water being pumped accumulates within the pumping mechanism. When either condition occurs, abnormally high currents can flow through the motor windings which can cause overheating and motor burn-out.
A variety of electrical controls have been devised in order to protect an electrical motor from damage caused by underload or overload conditions. It is known, for example, to detect an abnormal motor voltage or current as an indication of an overload or underload condition and to disconnect power from the motor in response thereto. Such a system is shown in Bello, et al., U.S. Pat. No. 4,584,623, wherein a circuit for protecting voltage sensitive motors, such as refrigerator compressors, detects an overvoltage or an undervoltage within the motor and disconnects the power source from the motor in response thereto. Likewise, Fry, et al., U.S. Pat. No. 4,875,000 discloses a circuit for detecting an overcurrent in a phase winding of a three phase motor and a mechanism, responsive to the detection circuit, for stopping the flow of current through the phase winding when an overcurrent occurs.
It is also known to detect an overload or an underload condition by sensing the power factor of the motor. For example, Miller, U.S. Pat. No. 4,703,387 discloses a circuit which measures the power factor of a motor by detecting a phase difference between a motor line voltage and a motor line current and a mechanism which disconnects the motor from a power supply when the power factor is outside a predetermined range. Gephart, et al., U.S. Pat. No. 4,123,792, discloses a system which multiplies a motor current by a power factor signal to produce a signal indicative of the instantaneous motor operating loads. This signal is then rectified, averaged and compared to a set of reference levels to detect an overload or an underload condition.
All of these systems are considered inadequate, however, because they rely on motor voltage, current or power factor measurements which are not as accurate an indication of the power delivered to the load as are motor input power measurements. Methods based on voltage or current measurements are also prone to inaccuracies caused by line or input voltage amplitude fluctuations.
Bejot, et al., U.S. Pat. No. 4,419,625 discloses a device for use in a three phase motor which integrates the product of a phase current signal, developed from one of the motor phase lines, with a line-to-line voltage signal, developed from the other two motor phase lines, to produce a power signal which represents the mean power absorbed by the motor. Signals representing motor losses are then subtracted from the power signal to produce an estimate of the power delivered to the load. This system, however, is complex and is susceptible to line voltage amplitude fluctuations which can cause inaccurate power readings.
It is a general object of the present invention to provide an improved motor protection system which utilizes simple and inexpensive components to sense motor input power and to detect motor overload and underload conditions. It is also a general object of the present invention to provide a unique motor energy measuring device which is more immune to line voltage amplitude fluctuations than known devices.